In my earlier U.S. Pat. No. 4,435,690, entitled "Primary Circuit Breaker" and issued on Mar. 6, 1984, and in my co-pending application, Ser. No. 06/572,913, filed on Jan. 23, 1984, and entitled "Trip Assembly For A Circuit Breaker" an under oil primary circuit breaker was disclosed for interrupting fault currents in the primary circuit of a transformer. These circuit breakers included an arc chamber which is immersed in the oil and is used to extinguish the arc produced between the interrupter contacts on current interrution. The primary circuit breaker is used to interrupt current under fault current conditions as well as to manually deenergize the transformer. If the oil in the transformer has dropped below the level of the arc chamber, the arc produced on interruption will be in an air medium and may not extinguish until major damage has been done to the transformer.